Because of their excellent mechanical characteristics, such as impact resistance, and because of their excellent heat resistance and transparency, polycarbonates are widely used for various purposes. Known as the method for the preparation of polycarbonates as referred to above is a process (interface method) which involves a direct reaction of aromatic dihydroxy compounds such as bisphenol with phosgene, or a process in which aromatic dihydroxy compounds such as bisphenol and carbonic diesters such as diphenyl carbonate are allowed in molten state to undergo ester interchange reaction (polycondensation reaction).
In a process for preparing polycarbonates by ester interchange reaction of aromatic dihydroxy compounds with carbonic diesters, usually the reactants are heated under reduced pressure at a temperature of 250.degree.-330.degree. C. and allowed in molten state to undergo the ester interchange reaction in the presence of catalysts such as organic acid salts, inorganic acid salts, oxides, hydroxides or hydrides of metals or alcoholates. This process is advantageous over the above-mentioned interface method in that polycarbonates can be prepared at a relatively low cost. In this process, however, the aromatic dihydroxy compound and the carbonic acid diester are reacted in molten state, and the polycarbonate being formed is exposed to elevated temperatures for a prolonged period time, and in consequence, there has been posed such a serious problem that the product assumes a yellow color, and becomes poor in heat resistance or water resistance. For this reason, polycarbonates prepared by this process have hardly found application in field such as plastic glass and lenses where materials having a good color tone are required.
Furthermore, Japanese Patent Laid-Open-to-Public Publn. No. 51719/1985 proposes a process for preparing polycarbonates, which involves the use of catalysts comprising nitrogen containing basic compounds and boron compounds, and the desired polycarbonates assuming a relatively pale color are obtained by the proposed process when the catalysts as proposed are used therefor. However, the proposed catalysts involved such a problem that they are low in polymerization activity.
With the view of solving such problems as mentioned above, we conducted extensive research and eventually have accomplished the present invention on the basis of our finding that the purpose intended can be attained by carrying out the polycondensation of aromatic dihydroxy compounds with carbonic diesters either using starting monomers having a reduced content of a specific impurity or in the presence of a specific catalyst.